A Method for Reducing the Singlet–Triplet Energy Gaps of TADF Materials for Improving the Blue OLED Efficiency

We have successfully synthesized a series of blue thermally activated delayed fluorescence emitters, BPy-pC, BPy-pTC, BPy-p2C, and BPy-p3C, bearing a 4-benzoylpyridine core as the electron-accepting unit and carbazolyl, tert-butylcarbazolyl, dicarbazolyl, and tercarbazolyl groups as the electron-donating units, respectively. The density functional theory calculation shows that all of the compounds have their lowest unoccupied molecular orbitals on the benzoylpyridine moiety. However, the highest occupied molecular orbital (HOMO) of BPy-p3C is widely dispersed to the whole tercarbazolyl group, while the HOMOs of BPy-pC and BPy-pTC are mainly on the carbazolyl and extended to the phenyl ring. As a result, ΔE ST is reduced from 0.29 eV for BPy-pC to 0.05 eV for BPy-p3C, and the organic light-emitting diodes using these materials as dopants emit blue light and their maximum external quantum efficiencies (EQEs) increase from 4.2% to 23.9% for BPy-pC and BPy-p3C, respectively. The EQE of the BPy-p3C-based device increases 2 times more than that of the BPy-pTC-based device without a significant change in the color coordinates.